Considering all the talk about it having a 13" display and the apparent delay of an entry-level mobile, I imagine that we'll be seeing a lot of changes for the ibook line.
In a perfect world:
-will stay with a 1" thick form factor (though hardware is shrinking, they could use the extra room to further reinforce a widescreen ibook)
-since it is widely rumored that a 13" screen will be added to the line, I don't think we are very far from seeing a 10 or 11" version.
-being that hardware has seen improvements since the recent ibooks release, I imagine consumers will have their choice of 60, 80, and 100gb (possibly even 120) HDD's...throwing out the 40gb option altogether. Reason I believe this, my understanding is that you have to designate parts of the HDD to either Windows or Mac OS.
-processors exceeding 1.8ghz twin core is doubtful
-integrated graphics is probable, but I could see a 64-128mb gpu being an option on the higher end model. Mac gaming is slowly becoming a reality, and since these mobiles will be running windows, I'd imagine that the gaming industry will want to keep up and begin producing what gamers want to see.
-improved wifi reception is guaranteed.
-may see a few of the originally powerbook-only options on the new ibooks, i.e. improved backlighting and keyboard illumination.
-8x superdrive is a given.
-probably an HD screen.
-Magsafe connector and a 10 hour battery.
-other miscellaneous improvements that will go overlooked, but will make all the difference in the world when it is released.
edit: 2gb of memory with a bus speed around 400mhz